In the magnetic recording method, as recording density becomes higher, the effect of external temperature and the like on a magnetic bit is more remarkable. Accordingly, a magnetic recording medium that has a higher magnetic coercive force is necessary, but when this type of recording medium is used, a higher magnetic field is required at the time of recording. The upper limit of the magnetic field that can be generated by the recording head depends on the saturation magnetic flux density, but the value the magnetic field is near the material limit, and a dramatic increase cannot be expected. To address this problem, a method has been proposed to ensure the stability of the recorded magnetic bit, which method is executed as follows: The recording medium is locally heated at the time of recording to cause magnetic weakening where the magnetic coercive force is small, and in that state magnetic recording is carried out before heating is stopped to allow natural cooling. This method is called the heat-assisted magnetic recording method.
In the heat-assisted magnetic recording method, heat is preferably momentarily applied to the recording medium. In addition, the mechanism for heating is not allowed to be in contact with the recording medium. For this reason heating is usually performed by utilizing light absorption, and the method using light for heating is called the light-assisted method. In the light-assisted method, in the case where ultra high density recording is carried out, the required spot diameter is about 20 nm, but because there is a diffraction limit in normal optical systems, the light cannot be focused to this extent.
To cope with this problem, a method has been proposed for obtaining a small spot diameter by irradiating a laser beam of a specific wavelength on the optical head in which silicon with high refractive index is used as the solid immersion lens (See Patent Document 1, for example).
In addition, a near field optical head that utilizes a near field beam generated at the optical opening of a size less than that of the irradiation beam wavelength is used, but there is a problem that the light efficiency of the conventional near field optical head is poor.
For this reason, a method has been proposed in which the utilization efficiency of light is improved by providing a lens and metal grating for plasmon generation on the transparent slider formed on the substrate (See Patent Document 2, for example).
In this method, a laser beam of a suitable wavelength is focused using the optical system and irradiated on a metal piece (called a plasmon probe) having a size of several tens of nm to generate near field light, and the near field light is used as a heating means.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2000-90475    Patent Document 2: Unexamined Japanese Patent Application Publication No. 2003-6913